Cheese compositions are generally prepared from dairy liquids by processes that include treating the liquid with a coagulating or clotting agent. The coagulating agent may be a curding enzyme, an acid, or a suitable bacterial culture or it may include such a culture. The coagulum or curd that results generally incorporates transformed casein, fats including natural butter fat, and flavorings that arise especially when a bacterial culture is used. The curd is usually separated from the liquid whey, which generally contains soluble proteins not affected by the coagulation; such proteins are, of course, not incorporated into the coagulum. The inability of whey proteins to be retained in the coagulum is an important factor contributing to a lack of efficiency in production of cheese curds, and to a reduction in overall yield. Failure to incorporate a significant amount of the protein solids that are present in the starting dairy liquids into the resulting cheese curds represents a significant loss of protein. These problems have been recognized for many years.
Several methods have been proposed early with the objective of recovering whey proteins in cheese products. For example, whey proteins have been concentrated or dried from whey, and then recombined with cheese (see, e.g., Kosikowski, Cheese and Fermented Foods, 2nd ed., Edwards Brothers, Inc., Ann Arbor, Mich., 1977, pp. 451-458). Unfortunately the cheese recovered from such procedures does not have the appropriate physical and chemical properties conducive to making good quality natural cheeses or process cheeses.
An alternative approach has been to coprecipitate whey proteins with casein, as disclosed, for example, in U.S. Pat. No. 3,535,304. Again, however, the final product of this process lacks the proper attributes for making processed and imitation cheeses.
A further attempt to incorporate whey proteins into cheese products used ultrafiltration of milk to concentrate all the components, such as the casein, the whey protein, and the butterfat, that do not permeate the ultrafiltration membrane. When such a composition is coagulated by contact with an acid or rennet, a curd forms. This curd, however, loses considerable quantities of the whey protein during compaction. An example of such a process is provided in U.S. Pat. No. 4,205,090 wherein the milk is concentrated to about one-fifth of its original volume. The resulting curd could only be used to provide soft cheeses such as Camembert or Roblechon. Hard cheeses, such as cheddar, Colby, and the like, could not be prepared using this product.
Ernstrom et al. (J. Dairy Science 63:2298-234 (1980)) described a process in which milk is concentrated to about 20% of the original volume by ultrafiltration, diafiltration, and evaporation. The resulting composition is then inoculated with a cheese starter to ferment the lactose and form a cheese base. The cheese base can be used to replace natural cheese components of process cheese. This process does not employ any renneting step to prepare a cheese curd.
Food processing methods employing transglutaminases have also been disclosed in recent years. For example, Japanese Patent 59059151 discloses treating an emulsion containing proteins, oils or fats, and water with transglutaminase to produce a gelatinous, crosslinked gel. Japanese Patent 02276541 discloses a food protein with a fiber texture having heat-resistance. The fiber texture is developed by treatment of a protein hydrogel with a transglutaminase in the presence of calcium ion to induce crosslinking of the surface of a fiber bundle. Japanese Patent 2131539 used transglutaminase to work on a fused cheese product containing milk solids to product a cheese food having a texture similar to boiled fish paste.
U.S. Pat. No. 5,156,956 discloses a transglutaminase purified from strains of the genus Streptoverticillium, as well as its chemical, physical, and enzymatic properties. This transglutaminase catalyzes formation of protein gelation products from protein solutions to produce conventional gel foodstuffs such as yoghurt, jelly, cheese, gel cosmetics, and the like. This method did not use transglutaminase and enzymatic clotting agents to produce cheese.
U.S. Pat. No. 5,356,639 discloses a process for the production of a fermented concentrate from milk, including whole milk, skim milk, and milk with added milk components. The concentrate could be used to make cheese. The process includes the steps of (1) selectively concentrating milk; (2) increasing the ionic strength of the concentrate to maintain the milk in the liquid phase (coagulum formation is prevented both during and after fermentation); (3) fermenting the concentrate with lactic acid producing bacteria; and (4) removing water from the fermented liquid concentrate. The final product includes substantially all of the whey proteins originally present in the milk.
U.S. Pat. No. 5,681,598 describes a process for producing cheese with a transglutaminase. The process includes (1) adding a transglutaminase to a milk or milk protein solution, (2) heat-treating the mixture, (3) adding a milk clotting enzyme for a fixed time, and (4) recovering a cheese. This process provides a large amount of cheese curd compared to conventional methods. Additionally, processes in which conventional cheese fermentation occurs first and transglutaminase treatment occurs subsequently, as well as simultaneous treatments, are disclosed. The milk clotting enzyme is preferably an animal rennet. Increases in total weight, but not in dry weight, of the curd when transglutaminase is used were observed.
U.S. Pat. No. 5,731,183 discloses a transglutaminase purified from strains of Bacillus subtilis, having particular physical and enzymatic characteristics, and a method for producing protein, peptide, or non-protein amino acid polymers that are cross-linked via their glutamine and lysine residues to form intermolecular or intramolecular conjugates. The transglutaminase may be used to produce crosslinked protein polymers that can be used in a variety of food substances including cheese. This reference differs from the instant disclosure in characterizing a bacterial transglutaminase while not disclosing process steps utilizing transglutaminase and clotting agents that are involved in producing cheese.
Banks et al. (Milchwissenschaft 42:212-215 (1987)) disclose that heating milk at temperatures from 95.degree. C. to 140.degree. C. and then acidifying permits a modest increase in protein content in the cheese upon Cheddar production. Unfortunately, the resulting cheese developed a bitter off-flavor in this process. Law et al. (Milchwissenschaft 49:63-37 (1994)) report that heat treatment of milk prior to cheddaring results in reduction of proteins in whey and/or in acid filtrates of the milk.
Han et al. (J. Agn. Food Chem. 44:1211-1217 (1996)) examined the activity of transglutaminase in forming heterologous dimers and trimers. It was found that .beta.-casein forms homopolymers whereas .beta.-lactoglobulin does not. In heterologous mixtures, transglutaminase was shown to catalyze dimer formation between .alpha.-lactalbumin and .beta.-casein but not between .beta.-casein and .beta.-lactoglobulin. Han et al. do not discuss any aspect of cheese production.
U.S. Pat. No. 5,523,237 discloses a plastein material which is defined as one made by reversing the activity of a protease enzyme (e.g., a serine protease) acting on proteinaceous material. The proteinaceous substrate is present at a concentration of 5-50%, and is preferably whey, casein, or soy protein. The enzyme preparation is substantially free of subtilisin A activity, and is specific for glutamic acid and aspartic acid residues. This protease is obtained from Bacillus licheniformis and is designated SP 446; its proteolytic activity is characterized in considerable detail. The viscosity of whey protein containing solutions is shown to increase as a result of the action of the enzyme.
International patent WO 93/22930 discloses treating milk with a transglutaminase (preferably mammalian activated Factor XIII) and then with an enzyme having milk clotting activity to provide a milk-like product. According to this publication, the product has microparticulated protein that has been aggregated by means of the enzyme with milk clotting activity, and has mouthfeel that resembles a fat emulsion. Preferably the milk clotting enzyme is a cheese rennet enzyme. This method, like that of U.S. Pat. No. 5,356,639, does not appear to provide a cheese curd.
International patent WO 94/21129 discloses a process for forming an acidified edible gel from milk. Transglutaminase is added to milk or a milk-like product, the pH is adjusted to 4.8 to 5.8, and the resulting composition is exposed to a heat treatment. The resulting edible gel is reported to have a pleasant consistency and mouthfeel.
International patent WO 94/21130 discloses a similar process for forming an edible gel from milk. Transglutaminase is added to milk or a milk-like product, rennet is then added, and the resulting composition is exposed to a heat treatment. Only a single phase gel (rather than separate curd and whey) was obtained. This gel is reported to have satisfactory organoleptic properties.
International patent WO 97101961 discloses a process for making cheese which retains proteins in the cheese. The milk is incubated with transglutaminase, followed by a treatment with a rennet to cause clotting and formation of a coagulate. After separating the whey from the coagulate, the coagulate is used to make cheese. The protein to be maintained in the cheese, as set forth in the description, relates to casein macropeptides that result from the action of the rennet, and that diffuse into the whey. This process differs from the instantly claimed invention in a number of ways. The process disclosed in this patent relates to the retention of casein macropeptides, rather than whey protein, in the cheese curd. Moreover, there is no requirement for an initial heating step, and the rennet employed in WO 97101961 is a conventional mammalian rennet.
Dybing et al. (J. Dairy Sci. 81:309-317 (1998)) postulated incorporating whey protein into cheese curd by concentrating the components, coagulating whey proteins using a variety of agents, and renneting a composition containing the coagulated whey protein and concentrated milk components. It was found, however, that none of the methods attempted succeeded in producing whey protein coagula that were recovered as cheese.
Guinee et al. (Int. Dairy Journal 5:543-568 (1995)) reviewed the state of the art relating to incorporation of whey protein into cheese. High-heat treatment of milk impairs rennet coagulation, curd syneresis, curd structure and texture, as well as functional properties such as meltability and stretchability. Guinee et al. discuss physical and chemical factors that may be responsible for these effects. In heat treatments that denature whey protein in milk compositions, they found that, in semi-hard cheeses that result from curding such treated compositions, the curd has higher whey protein levels, but also higher moisture level, lower pH value, poorer curd fusion, and lower yield (fracture) values during ripening.
In spite of many attempts documented over almost three decades of effort, there remains a need for a cheese curd with a significant incorporation of whey protein into the curd without significant reduction of organoleptic properties and for a method that significantly increases the incorporation of whey protein into cheese curd without adversely affecting the organoleptic and other properties of the resulting cheese. There further remains a need for cheese products prepared using excess whey protein that significantly increases retention of the whey protein, and for a method of making cheese products using excess whey protein that significantly increases the incorporation of whey protein into the cheeses. Additionally there remains a need for enhancing the yield and efficiency of making cheese with increased incorporation of whey protein into cheese products. The present invention addresses these long-felt needs.